The present invention relates to a method of mass spectrometry and a mass spectrometer.
It is known to employ Data Dependant Acquisitions (“DDA”) on a tandem mass spectrometer such as a quadrupole-Time of Flight mass spectrometer (“Q-ToF”). According to such known techniques the mass to charge ratios of parent or precursor ions are determined in an initial survey scan. A quadrupole mass filter is then arranged to sequentially isolate each individual parent or precursor ion in turn according to its mass to charge ratio and then to accelerate the selected parent or precursor ions into a collision cell in order to fragment the parent or precursor ions so as to produce product or fragment ions by Collision Induced Dissociation (“CID”).
The product or fragment ions are then mass analysed using a Time of Flight mass analyser. However, when parent or precursor ions are isolated by the mass filter then other parent or precursor ions will be discarded. As a result, the known mass spectrometer and method of operation suffers from a relatively low duty cycle.
The known method of selecting parent or precursor ions also suffers from the problem that it will result in some bias. For example, if the 20 most intense parent or precursor ions are selected then this will bias the data towards the most abundant species.
An improvement on the above described known approach is disclosed in U.S. Pat. No. 6,717,130 (Micromass) which discloses an approach wherein parent or precursor ions are not isolated and selected but rather fragment ions are assigned to parent ions by correlating their detection times to the times at which corresponding parent species elute from a chromatography column. This technique results in an improved duty cycle and minimises any bias in the acquisitions.
It is also known to operate a quadrupole-Time of Flight mass spectrometer by operating the quadrupole mass filter in a low-resolution mode with a transmission window of, for example, 25 Da. The mass to charge ratio range of the ions transmitted by the quadrupole mass filter is then sequentially incremented in steps of approximately 25 Da and in a manner that is not data dependant.
Ions exiting the quadrupole mass filter are accelerated into a gas cell so as to fragment by Collision Induced Dissociation and the resulting fragment ions are mass analysed by the Time of Flight mass analyser. The data from each 25 Da window is kept separate for processing. This technique is unbiased in the nature of the acquisition and has an improved duty cycle compared with other arrangements operating with narrower mass to charge ratio isolation windows.
The two Data Independent Acquisition (“DIA”) methods discussed above typically fragment parent or precursor ions using fragmentation methods that are particularly suited for fragmenting parent or precursor ions over a wide mass to charge ratio range. In particular, a Collision Induced Dissociation (“CID”) fragmentation cell is commonly used to fragment ions since Collision Induced Dissociation fragmentation is suitable for fragmenting ions having a wide range of mass to charge ratios.
However, fragmentation methods such as Collision Induced Dissociation tend to be limited in the types of fragmentation that they can induce and therefore in the fragment ion spectra that they can produce. As a result, this limits the information that can be obtained. For example, in the analysis of post translationally modified (“PTM”) peptides Collision Induced Dissociation methods give little information other than the loss of the side chain.
It is known that other types of fragmentation can advantageously induce different types of fragmentation in parent or precursor ions and can therefore produce different fragment ions from the same parent or precursor ion. These fragment ions can be useful in a number of applications. In particular, Electron Transfer Dissociation (“ETD”) is known to be particularly useful in the analysis of post translationally modified peptides.
However, seeking to fragment parent or precursor ions having potentially a wide mass to charge ratio range using an Electron Transfer Dissociation fragmentation device is problematic and fragmentation can be curtailed entirely due to an excessive amount of charge flowing through the Electron Transfer Dissociation reaction cell. This arises due to the necessity for there to be a surplus of reagent anions available to react with analyte cation charges or else the analyte cation charges tend to neutralise the reagent anions thereby rendering the Electron Transfer Dissociation reaction unviable.
It is therefore desired to provide and improved method of mass spectrometry and an improved mass spectrometer.